The invention generally relates to a system of and a method for fabricating semiconductor devices, and more particularly to a system, a device, and a method for improving lithographic critical dimension control.
With increasing sophistication and expertise in the fabrication of semiconductor devices, coupled with a demand for increasingly smaller die sizes, semiconductor device geometries, such as, for example, DRAM devices, are becoming smaller. An important limiting factor in decreasing semiconductor device geometries is accurate control of critical dimensions (CDs).
Errors in critical dimensions may be introduced to an exposure field on a wafer-in-process from reticle errors; from lithographic tools, such as dose nonuniformities or lens errors; from substrate variations causing systematic substrate reflectivity-induced errors, such as chemical-mechanical polishing dishing or doming or thin film deposition; and from systematic dry etch errors, such as process loading effects.
Another factor adding to the complexity of inhibiting critical dimension errors is that the transmission of light through a reticle may affect larger critical dimensions differently than smaller critical dimensions. Typically, smaller critical dimension features are more sensitive to the intensity level of light than larger critical dimension features.
Attempts have been made to address the issue of diminishing light intensity toward the edges of the exposure fields. See, for example, U.S. Pat. No. 6,021,009 (Borodovsky et al.). These attempts have been directed solely to making a more uniform transmission of light onto a wafer-in-process so that the light intensity experienced at the edge of the exposure field is similar to the light intensity experienced in the center of the exposure field.
There remains a need for a method of adjusting light intensity experienced across the exposure field of a wafer-in-process to accommodate varying sized critical dimensions due to long range mask critical dimension errors, systematic film thickness variations, and process loading effects, and for a device and system for locally adjusting the light intensity.
The invention provides a system for diminishing longer ranging critical dimension errors across each exposure field experienced on a wafer. The system includes a light source, a semiconductor mask including a die image for imaging across an entire wafer, and a pellicle. The mask and the pellicle are positioned between the light source and the wafer. The pellicle includes a light intensity modifying region configured to regulate the transmission of light from the light source onto the wafer to create a non-uniform light transmission profile across the entire wafer exposure field and thereby reduce critical dimension errors thereon.
The invention further provides a pellicle for diminishing critical dimension errors on a wafer. The pellicle includes a light intensity modifying region which is configured to regulate the transmission of light from a light source onto the wafer to create systematically a non-uniform light transmission profile across the exposure field and thereby reduce critical dimension errors thereon.
The invention also provides a method for reducing critical dimension errors on a wafer. One or more wafers are imaged and processed through any steps that will affect critical dimensions on the wafer, such as film depositions, planarization, etch steps or cleans. Critical dimensions are then measured across the exposure fields of the processed wafer(s). By comparing several exposure fields, this data allows the calculation of an average critical dimension error for each measurement point in the exposure field, which represents the correctable systematic component of the total critical dimension error. Extrapolation between measurement points allows the modeling of the critical dimension error distribution for all points in the exposure field. To compensate these errors, the local exposure intensity is modulated across the exposure field by a customized pellicle with locally variable light transmission. The required local pellicle transmission changes can be calculated using a correlation curve relating exposure dose to the resulting critical dimension, which can be measured in a separate calibration experiment, and a suitable pellicle can be manufactured for critical dimension corrected imaging.
The foregoing and other advantages and features of the invention will be more readily understood from the following detailed description of preferred embodiments, which is provided in connection with the accompanying drawings.